Energy & Fuels 2007, 21, 1601-1605
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Methane Cracking over a Bituminous Coal Char Zhi-qiang Sun,† Jin-hu Wu,*,† Mohammad Haghighi,‡ John Bromly,‡ Esther Ng,‡ Hui Ling Wee,‡ Yang Wang,† and Dong-ke Zhang‡ Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, People’s Republic of China, and Centre for Fuels and Energy, Curtin UniVersity of Technology, G.P.O. Box U1987, Perth WA 6845, Australia ReceiVed December 4, 2006. ReVised Manuscript ReceiVed March 29, 2007
Methane cracking over a bed of Chinese bituminous coal char was studied using a fixed-bed reactor at atmospheric pressure and temperatures between 1073 and 1223 K. Methane conversion over the fresh char increased with increasing temperature to 90% at 1223 K. Hydrogen was the only gas-phase product that was detected during the experimentation. The char was shown to exert a significant catalytic effect on methane cracking by comparing results from experiments with the raw char and demineralised char as well as from blank experiments using quartz. It was further shown that the ash was not the source of the catalytic effect of the char. However, both methane conversion and hydrogen yield decreased with increasing reaction time, irrespective of other experimental conditions, indicating that the char rapidly became deactivated following the exposure to methane. It was speculated that the deposition of carbon from methane cracking was responsible for this deactivation, which is supported by scanning electron microscopy (SEM) image analysis. It was demonstrated that the catalytic activity of the deactivated char can be partially recovered by burning off the carbon deposits with an oxidizing gas mixture containing 0.46% oxygen.
1. Introduction Research into methane reforming to produce syngas has attracted renewed interest in recent years. This is because ever more natural gas and coal-bed methane resources have been found on one hand and, on the other hand, there has been an increasing demand for syngas for the synthesis of ammonia, hydrocarbons via the Fischer-Tropsch process, methanol, and other chemicals, especially in China. Due to the high stability of methane molecules, the research into methane reforming has been focused on catalytic processes, especially on transition metal catalysts.1-4 However, one of the major problems in catalytic reforming of methane is that catalyst activity is greatly affected by carbon deposition. Another issue is the ratio of H2/CO produced from steam reforming of methane being theoretically 3, which is higher than usually required by various downstream synthesis processes. Coal has also been used in syngas production, and due to its high C/H ratio, the syngas produced has a low H2/CO ratio, just opposite to that of steam reforming methane. Therefore, combining coal gasification and methane reforming may serve as an option to produce syngas that meets the downstream processing requirements and may provide an effective means to utilize both coal and coal-bed methane with a significant environmental advantage.5 This is a very complex * Corresponding author. † Chinese Academy of Sciences. ‡ Curtin University of Technology. (1) Zhang, T. J.; Amiridis, M. D. Appl. Catal. A: Gen. 1998, 167, 161172. (2) Avdeeva, L. B.; Kochubek, D. L.; Shaikhutdinov, S. K. Appl. Catal. A: Gen. 1999, 177, 43-51. (3) Otsuka, K.; Kobayashi, S.; Takenaka, S. Appl. Catal. A: Gen. 2000, 190, 261-268. (4) Muradov, N. Int J. Hydrogen Energy 2001, 26, 1165-1175.
heterogeneous-homogeneous reaction system involving reactions such as: methane cracking
CH4 ) C(s) + 2H2 ∆H°298K ) 74.6 kJ mol-1
(R1)
steam and carbon dioxide reforming of methane
CH4 + CO2 ) 2CO + 2H2 ∆H°298K ) 247.1 kJ mol-1 (R2) CH4 + H2O(g) ) CO + 3H2 ∆H°298K ) 205.9 kJ mol-1 (R3) CH4 + 0.5O2 ) CO + 2H2 ∆H°298K ) -35.9 kJ mol-1 (R4) coal gasification by steam and carbon dioxide
C + CO2 ) 2CO ∆H°298K ) 159.7 kJ mol-1
(R5)
C + H2O ) CO + H2 ∆H°298K ) 118.9 kJ mol-1 (R6) C + 0.5O2 ) CO ∆H°298K ) -123.1 kJ mol-1 (R7) combustion reaction
CH4 + O2 ) CO2 + 2H2 ∆H°298K ) -802.9 kJ mol-1 (R8) C + O2 ) CO2 ∆H°298K ) -405.9 kJ mol-1
(R9)
(5) Wu, J. H.; Fang, Y. T.; Wang, Y.; Zhang, D. K. Energy Fuels 2005, 19, 512-516.
10.1021/ef060616v CCC: $37.00 © 2007 American Chemical Society Published on Web 05/02/2007
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water gas shift reaction
Sun et al. Table 1. Proximate and Ultimate Analysis of Char
CO + H2O(g) ) CO2 + H2 ∆H°298K ) -41.2 kJ mol-1 (R10) and possibly carbon deposition reaction
2CO ) C(graphite) + CO2 ∆H°298K ) -171.5 kJ mol-1 (R11) CO + H2 ) C(graphite) + H2O ∆H°298K ) -131.3 kJ mol-1 (R12) It has been further speculated that the coal char and/or its ash may exert a catalytic effect on methane reforming reactions,5 and if fully developed, this process will have at least three advantages: (i) the ratio of H2/CO of the syngas produced is adjustable; (ii) there is no need for more expensive metal catalyst as the coal char serves as the catalyst as well as a reactant, and carbon deposition on the catalyst (the char) is no longer a concern as it will gasify with O2, H2O, or CO2 and eventually consumed; (iii) the process is of high energy efficiency compared to the conventional catalytic steam reforming of natural gas. Furthermore, this process can be conveniently situated near coal mines where coal-bed methane is also readily available, providing an effective and economic means to utilize both coal and coal-bed methane resources at the same time. Following the success in proving the above concept of combining coal gasification and methane reforming in a labscale fluidized-bed reactor,5 detailed understanding of the science behind the working of the process is clearly necessary and a series of mechanistic studies are underway. One of the very first steps of our continuing research and development (R&D) effort is to investigate methane cracking over the char to reveal how methane and coal char interact. Recently, a number of studies of methane cracking on activated carbon have been reported with an aim to produce CO2-free hydrogen while the deposited carbon could be used as a fuel or carbon product. Muradov6 studied the catalytic effect of many carbon materials (such as active carbon, carbon black, and graphite) on methane cracking. It has been shown that activated carbon has the best catalytic activity while the catalytic activity of carbon black shows a linear dependence on its surface area. Kim et al.7 tested the catalytic activity of several Korea native activated carbons for methane cracking. The results showed that ash in the active carbons has little catalytic effect. The present work examines methane cracking on coal char at atmospheric pressure but different temperatures. In order to ascertain the catalytic effect of the char, quartz, coal ash, and demineralized char are also used in the experiments.
analysis
value
Proximate (wt %, ad: air-dry) moisture 0.38 volatile 1.32 ash 18.31 Ultimate Analysis (wt %, ad: air-dry) C 78.11 H 1.13 O 1.04 N 0.76 Stotal 0.27
cracking experiments. A thermocouple with a quartz sheath was inserted into the reactor to monitor the temperature. Methane and nitrogen (99.99% v/v) were used without further purification. The gas flow rate was controlled using a mass flow meter. Unless otherwise specified, the amount of char sample used in each experiment was ∼13 g for the raw char and ∼11 g for the demineralized char. In order to verify if the coal ash would exert any catalytic effect on methane cracking, two types of coal ash samples were also prepared by burning the char in a muffle furnace at 873 K and gasifying the char with steam at 1173 K, respectively, in the same fixed-bed reactor. The experiments were performed at temperatures from 1073 to 1223 K and atmospheric pressure was maintained throughout the experiments. In a typical experimental run, a char sample was weighed and placed into the reactor and heated in nitrogen. When a desired temperature was reached, the gas was switched to the reactant gas mixture containing 10% v/v methane in nitrogen. Allowing 3 min after the gas switching, gas samples from the exit stream of the reactor were taken periodically and analyzed using gas chromatographs fitted with molecular sieve columns and thermal conductivity detectors, using Ar as the carrier gas for H2 measurement and H2 for hydrocarbons, respectively. The total flow rate of the reactant gas was typically 250 mL min-1, unless otherwise specified. High dilution by nitrogen (90% N2) was arranged to ensure that the reactions took place under isothermal conditions. Nitrogen was also used as a balance for reactant conversion calculations. As nitrogen does not participate in any reactions in the present experiments, thus Nin ) Nout, (Nin is the input molecular number, and Nout is the output molecular number of nitrogen). Then, the balance of other species can be made directly by comparing the concentrations of them in the gas phase. There is no notable carbon-containing species detected in the gas phase (less than 0.1% v/v). The gas -solid contact time in the reactor was approximately 1 s. Ideally, methane would crack over the bed of char according to reaction (R1). Methane conversion (X) is expressed as X)
CCH4,in - CCH4,out CCH4,in
and the product yield was expressed as Y)
2. Experimental A Chinese coal (Binxian bituminous) was employed in this work. Raw char was prepared by devolatilising the coal in nitrogen in a fix bed reactor at 1173 K for 30 min. Results of the proximate and ultimate analysis of the raw char are presented in Table 1. Demineralized char was also prepared by washing the raw char with hydrochloric acid and hydrofluoric acid to less than 0.5% ash. All char samples were sieved to a size fraction of 0.355-0.63 mm. An electrically heated fixed-bed reactor made of quartz (22 mm i.d. and 400 mm length, respectively) was employed for methane (6) Muradov, N. Catal. Commun. 2001, 2, 89-94. (7) Kim, M. H.; Lee, E. K.; Jun, J. H.; et al. J. Hydrogen Energy 2004, 24, 187-193.
(1)
CH2,out 2CCH4,in
(2)
where “C” is the concentration of CH4 or H2 and the subscripts “in” and “out” refer to reactor inlet and outlet conditions, respectively.
3. Results and Discussion 3.1. Methane Cracking over Char and Quartz. It is wellknown that methane molecules are highly stable and could only crack or decompose at high temperatures, usually greater than 1473 K.8 In a blank experiment, methane conversion on a bed of quartz particles of the same size fraction as the char was tested with the results being shown in Figure 1. The highest
Coal Char Methane Cracking
Energy & Fuels, Vol. 21, No. 3, 2007 1603
Figure 3. Methane conversions at various reaction temperatures (N2: CH4 ) 9:1, total flow rate 250 mL min-1). Figure 1. Methane cracking over a bed of quartz (N2:CH4 ) 9:1, total flow rate 250 mL min-1).
Figure 4. Hydrogen yields at various reaction temperatures (N2:CH4 ) 9:1, total flow rate 250 mL min-1). Figure 2. Methane cracking over a bed of various mixtures of raw char and quartz at 1173 K (N2:CH4 ) 9:1, total flow rate 250 mL min-1).
value of methane conversion ever achieved over quartz particles is less than 1.5% within the present experimental temperature range. However, the methane conversion is observed to be as high as 60% over a bed of raw char at 1173 K as shown in Figure 2. Figure 2 also presents the results of methane cracking over a bed of different mixtures of char and quartz. It is clearly seen that as the proportion of char in the solid mix is reduced, the methane conversion also decreases, suggesting that it is the char, not quartz, that possess some catalytic effect on methane cracking. It is also noted that, at the same temperature, the catalytic effect of the char, as reflected by the high conversion levels, reduces very rapidly as the reaction proceeds and levels off after approximately 40 min when the methane conversions become fairly constant, as shown in Figure 2. Figures 3 and 4 display the results of methane conversion and corresponding hydrogen yield over the raw char at different temperatures. It is observable that the initial methane conversion increases sharply with increasing temperature. Hydrogen was the only gas-phase product detected in the experiments. The trend of hydrogen yield is similar to that of methane conversion. Although no other gas-phase product species were detected with the current analytical instrument (the GC), an examination of the ratio of hydrogen produced over methane reacted would be useful in the understanding the reactions that are taking place during the experiments. This ratio should always fall around 2. Figure 5 depicts the trend of this ratio at various reaction temperatures and as a function of the experimental time of methane cracking over a bed of raw char. Clearly, most of the data points are within the vicinity of 2, and this ratio does not (8) Gue´ret, C.; Daroux, M.; Billaud, F. Chem. Eng. Sci. 1997, 52, 815827.
Figure 5. Ratio of hydrogen produced over methane reacted at various temperatures and as a function of experimental time (N2:CH4 ) 9:1, total flow rate 250 mL min-1).
seem to change with either time or temperature, despite a decrease in methane conversion, which suggests that the overall reaction in the experiments follows the methane cracking reaction (R1). 3.2. Methane Cracking over Demineralized Char and Ash. Although the catalytic effect of the char on methane cracking has now been ascertained, it was not known if it is the fixed carbon or ash in the char that has the catalytic effect. Further methane cracking experiments were performed with the demineralized char and the coal ash. Figure 6 shows the results of methane cracking over the two types of coal ash produced from the same raw char. Neither the ash prepared by combustion in air nor the ash prepared by steam gasification shows any signs of catalytic effect on methane cracking as the methane conversion levels achieved in either case are only marginally higher than that of methane cracking over quartz shown in Figure 1. The methane conversion during cracking over the demineralized char is displayed in Figure 7, and the corresponding
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Figure 6. Methane cracking over the two different coal ash samples (N2:CH4 ) 9:1, total flow rate 250 mL min-1).
Sun et al.
Figure 9. Comparison of the measured initial methane conversion and hydrogen yield with thermodynamic equilibrium methane conversion
Figure 7. Methane conversion over a bed of demineralised char (N2: CH4 ) 9:1, total flow rate 250 mL min-1).
Figure 8. Hydrogen yields during methane cracking over the demineralized char (N2:CH4 ) 9:1, total flow rate 250 mL min-1).
hydrogen yields are displayed in Figure 8, together with the respective results for methane cracking over the raw char at 1073 and 1173 K. It is clear that the methane conversion over the demineralized char is only about 10% less than that over the raw char at 1173 K. This observation argues for the catalytic effect of the carbon in the char rather than the ash. Although the ash in the raw char is about 20% and contains metallic elements such as Fe and Mg, which are often considered to have a catalytic effect in methane reforming,9,10 they seem to do little in the methane cracking in the present experiments. 3.3. Thermodynamic Analysis. The methane cracking reaction (R1) is not thermodynamically favored and would require a high reaction temperature and/or an effective catalyst to facilitate it. The above experimental results have clearly demonstrated that Binxian coal char possesses the catalytic effect, promoting methane cracking. It would be of interest to (9) Ermakova, M. A.; Ermakov.; Yu, D. Catal. Today 2002, 77(3), 225235. (10) Choudhary, V. R.; Banerjee, S.; Rajput, A. M. J. Catal. 2001, 198 (1), 136-141.
Figure 10. Typical SEM images of fresh coal char (a) before methane cracking and (b) after 120 min of methane cracking at 1123 K (N2: CH4 ) 9:1, total flow rate 250 mL min-1).
know how far the experimentally observed methane conversion or hydrogen yield is from the thermodynamic limit. Figure 9 compares the results of the initial methane conversion measured in the experiments and the thermodynamic equilibrium of methane cracking obtained from ref 8. It is seen that the measured methane conversion is always below the thermodynamic expectations at respective temperatures and the difference becomes smaller as the reaction temperature increases. At the highest temperature of 1223 K in the present experimentation, the methane conversion reaches 90% of the equilibrium conversion, suggesting that the Binxian coal char is a very effective catalyst for methane cracking. 3.4. Deactivation and Regeneration of Catalytic Activity. Although the catalytic effect of the coal char on methane has been clearly demonstrated, the results presented so far consistently show a decreasing trend with increasing reaction time, indicating that the char rapidly loses its catalytic activity for methane. In order to probe the mechanisms of the char deactivation, a series of scanning electron microscopy (SEM)
Coal Char Methane Cracking
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that the catalytic activity of the char for methane cracking was partially recovered following the oxidation and the recovered catalytic activity increased with the regeneration time, as indicated by the initial H2 yields after regeneration, being 19%, 24%, and 30% for 10, 20, and 30 min, as shown in Figure 11. These preliminary results indicate that the deactivated char can restore its catalytic effect by burning off the carbon deposits after methane cracking. However, it should be noted that the char can also react with oxygen and is often more reactive than the carbon deposits. Therefore, any attempt to fully recover the char activity would be difficult, if not impossible. For the intended application of combined coal gasification and coal-bed methane reforming in a fluidized-bed reactor,5 the fresh char is continuously added to the reaction system and, as both oxygen and steam are present, the char is also subject to “continuous regeneration”. Therefore, a reasonable degree of char catalytic activity for methane cracking should be maintained in a real situation. Figure 11. Hydrogen yield over a bed of coal char before and after regeneration (N2:CH4 ) 9:1, total flow rate 200 mL min-1, 10.5 g char).
image analyses were performed on fresh char and char samples following methane cracking experiments at different temperatures and varying exposure times. It has been shown that there is a gradual carbon buildup on the char surface as the reaction time proceeds and the carbon deposition becomes more rapid as the temperature increases. Figure 10 shows typical SEM images of the fresh coal char and a char after 120 min of methane cracking at 1123 K and atmospheric pressure. From these SEM images, it can be seen that while the fresh char presents a firm and relatively clean surface, the deactivated char after methane cracking is covered with fluffy carbon deposits. The brighter appearance of the carbon deposits on the char as seen on the SEM image (Figure 10b) also suggests that the carbon deposit is of graphitic nature as graphite is conductive to the electron beam and, thus, looks brighter in the SEM. Clearly, the char deactivation can be mainly attributed to carbon deposition following methane cracking. Attempts to regenerate the catalytic activity of the char after periods of methane cracking and at different temperatures were then made by “burning off” the carbon deposits with highly diluted oxygen (ca. 0.5%) in nitrogen. Figure 11 presents typical results. In this experiment, cracking of 10% methane in nitrogen was first carried out at 1223 K for 180 min and it can be seen that the H2 yield decreased from the initial 63% to 12%. At this point, the reactant gas stream was switched to 0.46% O2 in nitrogen for 10, 20, and 30 min, respectively, to regenerate the catalyst, and then immediately switched back 10% methane in nitrogen. During the regeneration, no oxygen was detected in the exit stream, which means that all that oxygen in the feed gas had been consumed in carbon oxidation. The results show
4. Conclusions Methane cracking experiments have been performed over a bed of coal char, demineralized char, and ash as well as quartz in a fixed-bed reactor at temperatures ranging from 1073 to 1273 K. It has been clearly demonstrated that the char, rather than the ash, has a profound catalytic effect on methane cracking. It seems that methane cracking is the most dominant, if not the only, reaction taking place during the experiments, with hydrogen being the sole detectable gas product. Both methane conversion and hydrogen yield increase with increasing reaction temperature but decrease with reaction time, indicating that the char is deactivated following the exposure to methane. After 40-60 min, depending on the temperature, from the commencement of the experiments, the methane conversion and hydrogen yield become steady throughout the experimentation. The highest initial methane conversion of more than 75% is achieved with the raw char at 1223 K. This conversion is within 90% of the thermodynamic equilibrium expectation of methane cracking at this temperature. Carbon deposition from methane cracking has been shown to be primarily responsible for the loss of catalytic activity of the char. The catalytic activity of the deactivated char can be partially recovered by burning off the carbon deposits with diluted oxygen. Acknowledgment. This work has been jointly funded by National Natural Science Foundation of China (NSFC) under the Young Overseas Chinese Scholars scheme (grant number 50628404), the Innovation Funds Scheme of the Institute of Coal Chemistry, Chinese Academy of Sciences (ICC, CAS), and the Outstanding Overseas Chinese Talent Funds Scheme of Chinese Academy of Sciences (CAS). EF060616V
